# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None


class ListNode(object):
     def __init__(self, x):
        self.val = x
        self.next = None


class Solution(object):
    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        if l1 is None:
            return l2
        if l2 is None:
            return l1
        if l1.val < l2.val:
            head = ListNode(l1.val)
            head.next = self.mergeTwoLists(l1.next, l2)
            return head
        else:
            head = ListNode(l2.val)
            head.next = self.mergeTwoLists(l1, l2.next)
            return head


